Unbreak lock conflict detection for Hot Standby.

This got broken in the original fast-path locking patch, because
I failed to account for the fact that Hot Standby startup process
might take a strong relation lock on a relation in a database to
which it is not bound, and confused MyDatabaseId with the database
ID of the relation being locked.

Report and diagnosis by Andres Freund.  Final form of patch by me.

src/backend/storage/lmgr/lock.c

@@ -2445,8 +2445,8 @@ FastPathTransferRelationLocks(LockMethod lockMethodTable, const LOCKTAG *locktag
 		LWLockAcquire(proc->backendLock, LW_EXCLUSIVE);
 
 		/*
-		 * If the target backend isn't referencing the same database as we
-		 * are, then we needn't examine the individual relation IDs at all;
+		 * If the target backend isn't referencing the same database as the
+		 * lock, then we needn't examine the individual relation IDs at all;
 		 * none of them can be relevant.
 		 *
 		 * proc->databaseId is set at backend startup time and never changes
@@ -2459,7 +2459,7 @@ FastPathTransferRelationLocks(LockMethod lockMethodTable, const LOCKTAG *locktag
 		 * fencing operation since the other backend set proc->databaseId.	So
 		 * for now, we test it after acquiring the LWLock just to be safe.
 		 */
-		if (proc->databaseId != MyDatabaseId)
+		if (proc->databaseId != locktag->locktag_field1)
 		{
 			LWLockRelease(proc->backendLock);
 			continue;
@@ -2677,14 +2677,14 @@ GetLockConflicts(const LOCKTAG *locktag, LOCKMODE lockmode)
 			LWLockAcquire(proc->backendLock, LW_SHARED);
 
 			/*
-			 * If the target backend isn't referencing the same database as we
-			 * are, then we needn't examine the individual relation IDs at
+			 * If the target backend isn't referencing the same database as the
+			 * lock, then we needn't examine the individual relation IDs at
 			 * all; none of them can be relevant.
 			 *
 			 * See FastPathTransferLocks() for discussion of why we do this
 			 * test after acquiring the lock.
 			 */
-			if (proc->databaseId != MyDatabaseId)
+			if (proc->databaseId != locktag->locktag_field1)
 			{
 				LWLockRelease(proc->backendLock);
 				continue;